The present invention relates to a process for pretreating readily passivated metals, notably stainless steels and superalloys, for electroplating metals, such as chromium, nickel and cobalt, to produce deposits of greatly improved adhesion to the substrate.
The utility of superalloys in some defense oriented applications can be greatly improved by electroplating a coating of a metal, such as chromium, to change the surface properties of the alloy. However, superalloys, like stainless steels, are readily passivated by the formation of surface oxide films. Such films present special problems for electroplating, since they can reduce deposit adhesion, and special cleaning and processing is required to remove the films prior to plating, as is well known in the art.
Various methods are known for activating stainless steels for electroplating. These methods are generally discussed in ASTM Designation B254-53, "Preparation of and Plating Stainless Steels", September (1964), and include (a) cathodic or anodic treatments in alkaline or acidic solutions. (b) immersion treatments and (c) simultaneous activation using a strike intermediate layer. A strike layer of intermediate metal such as nickel, cobalt, zinc and cadmium is commonly employed to protect the stainless steel after activation; and since the strike layer itself is applied by electroplating, at least two electroplating steps are required in the process. Further, the effectiveness of the activation and strike layer decreases as the chromium content of the stainless steel increases.
Superalloys are alloys developed for very high temperature service where relatively high stresses are encountered and where oxidation resistance is frequently required (Metals Handbook 8th Edition Vol. 1, page 37). They are generally nickel, cobalt or iron-base alloys wherein the high temperature properties are derived through solid solution strengthening and hardening by precipitated metal carbide and gamma prime phases. Although superalloys are similar to stainless steels, it is not known whether the same activation methods are suitable for both. The few references on superalloys suggests the need for an intermediate strike layer in a viable activating process. However, the presence of the intermediate strike layer can exclude the electroplated alloy from service in high temperature environments.
It is an object of the present invention to provide a method for pretreating superalloys and stainless steels which overcomes the aforementioned problems and eliminates the need for an intermediate strike layer.